1. Field of the Invention
The invention relates to light emitting diodes (LED or LEDs) and LED components having primary optics for emission pattern shaping, and in particular for emission pattern broadening.
2. Description of the Related Art
Different LED light fixtures have been developed such as troffer-style fixtures that are used in commercial office and industrial spaces throughout the world. Previous troffers housed elongated fluorescent light bulbs that span the length of the troffer. Troffers may be mounted to or suspended from ceilings, such as being suspended by a “T-grid”. Often the troffer may be recessed into the ceiling, with the back side of the troffer (i.e. troffer pan) protruding into the plenum area above the ceiling a distance of up to six inches or more. In other arrangements, elements of the troffer on the back side dissipate heat generated by the light source into the plenum where air can be circulated to facilitate the cooling mechanism. U.S. Pat. No. 5,823,663 to Bell, et al. and U.S. Pat. No. 6,210,025 to Schmidt, et al. are examples of typical troffer-style fixtures. These fixtures can require a significant amount of ceiling space to operate properly.
More recently, with the advent of the efficient solid state lighting sources, these troffers have been used with solid state light sources, such as light emitting diodes (LEDs). LEDs are solid state devices that convert electric energy to light and generally comprise one or more active regions of semiconductor material interposed between oppositely doped semiconductor layers. When a bias is applied across the doped layers, holes and electrons are injected into the active region where they recombine to generate light. Light is produced in the active region and emitted from surfaces of the LED.
LEDs have certain characteristics that make them desirable for many lighting applications that were previously the realm of incandescent or fluorescent lights. Incandescent lights are very energy-inefficient light sources with approximately ninety percent of the electricity they consume being released as heat rather than light. Fluorescent light bulbs are more energy efficient than incandescent light bulbs by a factor of about 10, but are still relatively inefficient. LEDs by contrast, can emit the same luminous flux as incandescent and fluorescent lights using a fraction of the energy.
In addition, LEDs can have a significantly longer operational lifetime. Incandescent light bulbs have relatively short lifetimes, with some having a lifetime in the range of about 750-1000 hours. Fluorescent bulbs can also have lifetimes longer than incandescent bulbs such as in the range of approximately 10,000-20,000 hours, but provide less desirable color emission. In comparison, LEDs can have lifetimes between 50,000 and 70,000 hours. The increased efficiency and extended lifetime of LEDs is attractive to many lighting suppliers and has resulted in LED light sources being used in place of conventional lighting in many different applications. It is predicted that further improvements will result in their general acceptance in more and more lighting applications. An increase in the adoption of LEDs in place of incandescent or fluorescent lighting would result in increased lighting efficiency and significant energy saving.
One design challenge associated with LED-based multi-source luminaires is blending the light from LED sources within the luminaire so as to provide a uniform illuminance and color uniformity across the light emitting surfaces of the luminaire. This is particularly true in the case of luminaires in which the LED sources are “forward facing”—i.e., oriented with the peak emission intensity directed towards the light emitting surfaces of the luminaire (in contrast to an “indirect” orientation in which the majority of light is reflected or scattered off of one or more surfaces prior to reaching the light emitting surface of the luminaire). FIGS. 1 and 2 show a conventional LED component 10 that can comprise many different commercially available LED components such as the XP family of LED components commercially available from Cree, Inc. The components can be used in current “forward facing” luminaire, with the component 10 generally comprising a submount or printed circuit board (PCB) 12, LED chip 14, and a primary optic or lens 16. In most conventional LED components, the primary optic is in a relatively simple shape such as hemispheric. These conventional components typically produce a Lambertian or cosine-like beam emission profiles as shown in graphs 18 and 20 FIGS. 3 and 4, respectively. These emission profiles are relatively narrow with a peak that can result in undesirable near-field intensity and color variations across the emitting surface of the luminaire.
FIG. 5 shows one embodiment of a troffer 22 having conventional LED components oriented in a “forward emitting” configuration, along with the simulated resulting illuminance pattern expected on the luminaire emission surface 24 (in order to simplify the simulation, the LED components occupy only approximately half of the luminaire). The Lambertian emission pattern of the LED components results in undesirable hot spots in intensity along the troffer's emission surface 24, with FIG. 6 being a graph 26 showing the relative peaks of the emission hot spots. Such a non-uniform luminance distribution is generally considered to be aesthetically undesirable for most lighting applications.
Accordingly, standard LED components with standard primary optics may not be compatible with forward facing or direct backlighting. Secondary optics have been developed for these components that can be used to shape the emission pattern to reduce the hot spots. Secondary optics, however, are formed separately from the LED components and are bonded to the LED component's primary optic. This approach results in additional manufacturing complexities and costs, and can also result in efficiency losses due to both absorption in the secondary optic and Fresnel loss associated with the additional optical interfaces presented by the secondary optic.
The emission surface of a direct or front-facing LED-based luminaire typically comprises a sheet of material which may incorporate scattering particles or surfaces or a surface texture to help mask or minimize luminance variations. However, in order to have a significant effect on the luminance distribution, it is generally necessary to provide a high degree of scattering—typically resulting in each light ray emitted by the LED component having multiple encounters with various internal surfaces of the luminaire. This approach can lead to a significant loss in efficiency since such scattering surfaces are not ideal and each scattering event thus leads to light loss.
Some recent designs have incorporated light sources or light engines utilizing an indirect lighting scheme in which the LEDs or other sources are aimed in a direction other than the intended emission direction. This may be done to encourage the light to interact with internal elements, such as scattering surfaces, for example. One example of an indirect fixture can be found in U.S. Pat. No. 7,722,220 to Van de Ven which is commonly assigned with the present application. As in the case of direct view luminaires with heavily diffusing elements, interaction with multiple scattering elements within the luminaire body can result in optical losses, with the losses increasing with the number of interactions.